In order to fuse electroscopic toner material permanently onto a support surface by heat, it is usually necessary to elevate the temperature of the toner material to a point at which the constituents of the toner materials coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to become firmly bonded to the support member.
The use of thermal energy for fixing toner images onto a support member is well known. Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means, for example, a roll pair maintained in pressure contact, a flat or curved plate member in pressure contact with a roll, and a belt member in pressure contact with a roll.
Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. Typically, in such direct contact systems, the roller surface may by dry, i.e. no application of a release agent to the surface of the roller as described, for example, in U.S. Pat. Nos. 3,498,596 and 3,666,447. Alternatively, the fuser roll surface may be wetted with a release agent such as a silicone oil as described in U.S. Pat. Nos. 3,268,351 and 3,256,002. It is also known in the art to fuse toner images by the use of a flash fusing process, for example, as disclosed in U.S. Pat. No. 3,874,892. In such a process, a flash lamp is generally pulsed on for a very short period of time. It can be appreciated that since the lamp is pulsed or flashed for short period of time, a large amount of power must be used to accomplish the fusing of the toner particles.
Another method for fusing toner images to a substrate is radiant fusing. Radiant fusing differs from flash fusing in that in radiant fusing, the radiant energy source, typically and infrared quartz lamp, are turned on during the entire fusing step rather than pulsed for a short period of time as in flash fusing. Examples of radiant fuser apparatus are shown in U.S. Pat. Nos. 3,898,424 and 3,953,709. Such prior art radiant fusers are generally made of relatively heavy metallic construction which requires the constant use of a heating element to maintain the apparatus at standby temperature. U.S. Pat. No. 3,471,683 shows a heater roll with a printed circuit heating element. However, the heater roll is relatively thick and the adhesive material not suitable for relatively high temperature operation.
Such prior art fusing systems have been effective in providing the fusing of many copies in relatively large, fast duplicating machines, in which the use of standby heating elements to maintain the machine at or near its operating temperature can be justified. However, there is a continuing need for an instant-on fuser which requires no standby power for maintaining the fuser apparatus at a temperature above the ambient. It is known to use a positive characteristic thermistor having a self temperature controlling property as a heater for a heating roller. The roller is regulated to a prescribed temperature by a heating control temperature detection element. It is known to employ radiation absorbing materials for the fuser roll construction to effect faster warm-up time as described in U.S. Pat. No. 3,669,706. It is also disclosed in U.S. Pat. No. 4,355,255 to use an instant-on radiant fuser apparatus made of a low mass reflector thermally spaced from a housing, with the housing and the reflector together forming a conduit for the passage of cooling air therein. A low mass platen is provided which is constructed to achieve an operating temperature condition in a matter of a few seconds without the use of any standby heating device. It is also known as disclosed in U.S. Pat. No. 3,948,214 to use a cylindrical member having a first layer made of elastomeric material for transporting radiant energy, a second layer for absorbing radiant energy, and a third layer covering the second layer to affect a good release characteristic on the fuser roll surface. The fuser roll layers are relatively thin and have an instant-start capability. U.S. Pat. No. 4,395,109 discloses an instant-on fuser having a core of metal or ceramic supporting a fuser roller, and including a heat insulating layer, an electrically insulating layer and a protective layer formed on the outer circumference of the core.
A difficulty with the prior art fusing systems is that they are often relatively complex and expensive to construct and/or the mass of the system is relatively large to preclude an instant-start fusing capability. Another difficulty is that prior art fuser rolls are not always easily adapted to provide sufficient mechanical strength depending upon the size of paper to be fused or able to be tailored to selectively fuse different size copy sheets. It is an object of the present invention, therefore, to provide a new and improved instant-on fusing apparatus. It is another object of the present invention to provide an instant-on fuser apparatus that has a relatively low thermal mass and is designed for relatively ease of construction. It is another object of the present invention to provide a relatively high mechanical strength instant-on fuser roll regardless of the size of the copy sheet to be fused and to provide a fuser roll that can selectively fuse different size copy sheets.
Further objects and advantages of the present invention will become apparent as the following description proceeds and the features of novelty characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.